1. Field of the Invention
The present invention relates to a heating element for heating crucibles as disclosed in claims 1, 7 and 17 as well as to an arrangement of heating elements for heating crucibles as disclosed in claim 13. In particular, these heating elements can be used for heating a crucible when growing semiconductor crystals such as silicon, germanium and gallium arsenide.
2. Prior Art
A known device for growing semiconductor crystals using the Liquid Encapsulated Czochralski (LEC) process is schematically shown in FIG. 7. Such device is known for example from Ishiki, M., Recent Development of Bulk Crystal Growth, Research Signpost 1998, pages 63-64 (ISBN: 81-86481-58-3). A crucible 1 for receiving a molten mass 2 of semiconducting material is arranged on a crucible holder 3 within a cooled recipient 4. The molten semiconductor 2 is covered with a molten mass of boron oxide 12 to prevent volatile components (e.g. As in GaAs melts) from escaping by evaporation. To that end, the steam pressure in the recipient 4 has to be greater than the steam pressure of the volatile component (e.g.  greater than 2 bars in the case of GaAs). For heating the crucible 1, there is provided an arrangement of heating elements consisting of a lower heating element 100 surrounding the crucible 1 at its bottom and in a wall region adjacent to the bottom, and an upper heating element 200 provided above the lower heating element 100 and surrounding the upper region of the crucible wall. The lower heating element 100 is connected via power leads 110 to electrodes 120 provided at the bottom of the recipient and the upper heating element 200 is connected via power leads 210 to electrodes 220 which are also provided at the bottom of the recipient. A crystal nucleus 5 is connected to a pulling device 6 fed through the cover of the recipient 4 and arranged above the crucible 1. Also, a growing crystal 7 being pulled from the moltem mass 2 is connected to the pulling device 6 via said crystal nucleus 5. A drive is provided for rotating the crucible holder 3 and the crystal nucleus 5 in opposite directions during the growing process. The arrangement of heating elements 100, 200 is thermally insulated by an annular insulating pipe 8 surrounding the heating elements and consisting of a thermally insulating material. Only a small gap is provided between the insulating pipe 8 and the heating elements 100, 200 in order to prevent excessive gas convection which, due to the high pressure within the recipient, may result in large temperature fluctuations in particular with semiconductor materials having a highly volatile component such as gallium arsenide or indium phosphide.
U.S. Pat. No. 4,533,822 discloses a tulip-shaped heating element used as a bottom heater, which heating element is formed as a jacket-type hollow body having an upper edge and a lower edge, said hollow body comprising a first hollow cylindrical portion having a first cross section and a second portion adjacent thereto and having a tapering cross section, for surrounding a bottom region of a crucible.
DE 44 23 196 A1 discloses a hollow cylindrical heating element 200 for heating crucibles, having an upper edge and a lower edge, and comprising at least one power lead extending from said lower edge as an extension of the cylinder wall of said hollow body.
However, a combination of the known tulip-shaped heating element 100 and the hollow cylindrical heating element 200 in the vertical arrangement shown in FIG. 7 requires an interruption of the tubular thermal insulation with the insulating pipe 8 at least at two openings 9, 10 for feeding through the power leads 210 to the upper heating element 200. Through these openings 9, 10, a cold gas may flow in from the space surrounded by the cooled recipient 4, thus affecting crystal growth.
The vertical arrangement of two heating elements as shown in FIG. 7 enables variable adjustment of the axial temperature gradient within the inner space. However, an abrupt thermal transition at the gap 11 between the lower heating element 100 and the upper heating element 200 causes a disturbance of the axial temperature distribution in this region as well as unfavorable mechanical stresses in the growing single crystal, reducing the yield of single crystal material.
It is an object of the present invention to provide a heating element and an arrangement of heating elements for heating a crucible, in particular for growing semiconductor single crystals, preventing disturbances in the desired temperature distribution in the region of the molten material and the growing crystal and improving its yield.
This object is achieved by a heating element according to claims 1, 7 or 17 and by an arrangement of heating elements according to claim 13. Further improvements of the invention are presented in the dependent claims.
The invention will be better understood from the following description and figures of preferred embodiments which are presented for illustrative purposes only and are not to be construed as limiting the scope of the present invention.